ADT7475 Analog Devices, Inc., ADT7475 Datasheet - Page 33

• Manufacturer Part Number: ADT7475
• Description: Dbcool Remote Thermal Monitor And Fan Controller
• Manufacturer: Analog Devices, Inc.

PROGRAMMING THE AUTOMATIC FAN SPEED CONTROL LOOP
To more efficiently understand the automatic fan speed control
loop, it is strongly recommended to use the ADT7475 evalua-
tion board and software while reading this section.
This section provides the system designer with an understanding
of the automatic fan control loop, and provides step-by-step
guidance on effectively evaluating and selecting critical system
parameters. To optimize the system characteristics, the designer
needs to give some thought to system configuration, including
the number of fans, where they are located, and what tempera-
tures are being measured in the particular system.
The mechanical or thermal engineer who is tasked with the
system thermal characterization should also be involved at
the beginning of this process.
AUTOMATIC FAN CONTROL OVERVIEW
The ADT7475 can automatically control the speed of fans based
upon the measured temperature. This is done independently of
CPU intervention once initial parameters are set up.
The ADT7475 has a local temperature sensor and two remote
temperature channels that can be connected to a CPU on-chip
thermal diode (available on Intel Pentium class and other
CPUs). These three temperature channels can be used as the
basis for automatic fan speed control to drive fans using pulse-
width modulation (PWM).
Automatic fan speed control reduces acoustic noise by
optimizing fan speed according to accurately measured
temperature. Reducing fan speed can also decrease system
current consumption.
REMOTE 1
REMOTE 2
LOCAL
TEMP
TEMP
TEMP
THERMAL CALIBRATION
THERMAL CALIBRATION
THERMAL CALIBRATION
T
T
T
MIN
MIN
MIN
T
T
T
RANGE
RANGE
RANGE
Figure 43. Automatic Fan Control Block Diagram
100%
100%
100%
0%
0%
0%
MUX
Rev. B | Page 33 of 68
PWM
PWM
PWM
MIN
MIN
MIN
The automatic fan speed control mode is very flexible owing to
the number of programmable parameters, including T
T
and, therefore, for a given fan are critical because they define
the thermal characteristics of the system. The thermal vali-
dation of the system is one of the most important steps in the
design process, so select these values carefully.
Figure 43 gives a top-level overview of the automatic fan control
circuitry on the ADT7475. From a systems-level perspective,
up to three system temperatures can be monitored and used to
control three PWM outputs. The three PWM outputs can be used
to control up to four fans. The ADT7475 allows the speed of four
fans to be monitored. Each temperature channel has a thermal
calibration block, allowing the designer to individually configure
the thermal characteristics of each temperature channel.
For example, the designer can decide to run the CPU fan when
CPU temperature increases above 60°C and a chassis fan when
the local temperature increases above 45°C. At this stage, the
designer has not assigned these thermal calibration settings to a
particular fan drive (PWM) channel. The right side of Figure 43
shows controls that are fan-specific. The designer has individual
control over parameters such as minimum PWM duty cycle, fan
speed failure thresholds, and even ramp control of the PWM
outputs. Automatic fan control, then, ultimately allows graceful
fan speed changes that are less perceptible to the system user.
RANGE
CONFIG
CONFIG
CONFIG
TACHOMETER 1
MEASUREMENT
TACHOMETER 2
MEASUREMENT
TACHOMETER 3
MEASUREMENT
PWM
PWM
PWM
. The T
GENERATOR
GENERATOR
GENERATOR
AND 4
PWM
PWM
PWM
MIN
and T
ENHANCEMENT)
ENHANCEMENT)
ENHANCEMENT)
RANGE
(ACOUSTIC
(ACOUSTIC
(ACOUSTIC
CONTROL
CONTROL
CONTROL
RAMP
RAMP
RAMP
values for a temperature channel,
PWM1
TACH1
PWM2
TACH2
PWM3
TACH3
ADT7475
MIN
and